Note: There are 2 cables available for the RTS.
1) The 3.5mm plug is for Android or basic MP3 players. Downside is you do not get full control of the player. You can only advance back or forward a song with the handlebar controls.
2) The iPhone/iPod 36 pin plug. This is for Apple only. Apple products have an advanced control protocol no others have. It allows full control of the device. Playlists, artist, title, etc etc...
If you want full capability which is real nice, I recommend getting an older Apple iPod like the iPod touch 4th gen and leave it dedicated in the trunk. You can get a 4th gen 32GB for less than $50 if you shop around on Ebay.
This video shows you the capabilities of having an iPod.
Bob